selftests/powerpc: Add Anton's null_syscall benchmark to the selftests

Pull in a version of Anton's null_syscall benchmark:
  http://ozlabs.org/~anton/junkcode/null_syscall.c



Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Anton Blanchard <anton@au.ibm.com>
Signed-off-by: Rui Teng <rui.teng@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>